Method of producing semi-conducting glaze compositions

ABSTRACT

A semi-conducting glaze composition is prepared by calcining a mixture of 85 - 94 mol percent, calculated as SnO2, of tin oxide and 6 - 15 mol percent, calculated as Sb2O5, of antimony oxide at a temperature of 1,000* - 1,300*C under an oxidizing atmosphere; mixing 25 - 45 percent by weight of the calcined material with 55 - 75 percent by weight of a conventional ceramic glaze composition; melting the resulting mixture at a temperature of 1,200* - 1,400*C under an oxidizing atmosphere and pulverizing the melt to prepare a fritted material; and mixing again not less than 70 percent by weight of the resulting fritted material with not more than 30 percent by weight of at least one member selected from the group consisting of clay, kaolin, bentonite and conventional ceramic glaze composition. By applying the abovementioned semi-conducting glaze composition on a prepared insulator body, the surface electrical stress distributions of a heavy duty suspension and a solid core insulators, which require the firing under a reducing atmosphere, can be improved.

atnt [1 1 Ogawa et a1.

METHOD OF PRODUCING SEMH-CONDUQTHNG GLAZE COMPOSITIONS [75] inventors: Yutaka Ogawa, Nagoya; Takayuki Ogasawara, Kohnan; Shoji Seilre, Nagoya, all of Japan [73] Assignee: NGK Insulators, Ltd., Nagoya City,

Japan [22] Filed: Mar. 114, 1972 [21] Appl. No.: 234,594

Related U.S. Application Data [62] Division of Ser. No. 70,404, Sept. 8, [970, Pat. No.

[30] Foreign Application Priority Data Oct. 11, 1969 Japan 44-81014 [52] US. Cl 65/60, 65/134, 117/229 [51] C03c 17/04 [58] Field of Search 106/46, 72; 65/134, 60; 117/229 [56] References Cited UNITED STATES PATENTS 2,917,394 12/1959 Schurecht 106/46 3,368,026 2/1968 Vince 106/46 X 9,795,999 Mar. 5, 1974 3,523,028 Prokopowitz 106/46 X [57] ABSTRACT A semi-conducting glaze composition is prepared by calcining a mixture of 85 94 mol percent, calculated as SnO of tin oxide and 6 mol percent, calculated as Sb O of antimony oxide at a temperature of 1,000 1,300C under an oxidizing atmosphere; mixing percent by weight of the calcined material with 75 percent by weight of a conventional ceramic glaze composition; melting the resulting mixture at a temperature of 1,200" 1,40()C under an oxidizing atmosphere and pulverizing the melt to prepare a fritted material; and mixing again not less than percent by weight of the resulting fritted material with not more than 30 percent by weight of at least one member selected from the group consisting of clay, kaolin, bentonite and conventional ceramic glaze composition. By applying the above-mentioned semiconducting glaze composition on a prepared insulator body, the surface electrical stress distributions of a heavy duty suspension and a solid core insulators, which require the firing under a reducing atmosphere, can be improved.

4 Claims, 5 Drawing Figures (A) Conventional method Stannlc oxide 99-95mol v.) g l 985-95mol Colcinatl Mix'ng m moo i 25-45WY /a I El I Anti mon moo-c or Puverim'lo" Semi-conductin Firing under h some msu aror m 9 0 and mu --Gtazinq on oxidizing (l-5 mol l. or) Conwntional cercomposition atmosphere condfcmg 0,5-5mol I. on t rc glaze compocootmq SI On 55 -vrt 'l. or 75-ET5 wt ls l (B) The method of the present invention Tin component 94-86 mol 7o calculated as SD02 Colcinution or 237mm l00O-l300C a Ammo" Mums *gffi l f (asfliswt Pub/Grimm" Melting and frittlnq compong m 0 C I mixing -l 00C Electric insulator atmosphere N I 6-l5mol I. carom|c glaze undonolr or an huvlnq a semicolculoted as l Oxidllmtl tconducting glaze Sb Og (55 -75 wt Al coating Fritted material (not less than H mm Isl Pulvarizatiun 23:2,, wndwmq Flrmq l and mixing 'W W. lay,koolm, bentomta and/or composmon atmosphere conventional ceramic glaze compooitiono (not mom than 30 wt PAIENIEBRAR 515m Surface resistivity (MQ/sq) SHEEJ 2 BF 4 Surface condition of semiconducting glaze coating 500 blistered I good IOO l l l 1 82 8O (Sn02) O 2 4 6 8 IO l2 l4 l6 l8 20(Sb205) Ratio of tin component'to antimony component (moi%, calculated as Sn02 and Sb205) PATENTEDHAR Sign 3.795.499

SHEEI 3 0f 4 Fig. 3

700 3 CO gas atmosphere '00 cur atmosphere Surfuce resistivity (Mn/sq) 1 1 1 l 1 l I000 "00 I200 I300 I400 I500 Fritting temperature (C) PAIENTEDIAR mm samuorq I (Electrically IO I5 3O 4O 5O conducflve oxide 90 85 8O 7o 5 0 55 50 45 (COHVQHIOHOLQGHJITNC) b O O 7 glaze composmon Mixing ratio by weight) METHOD OFIPRODUCING SEMI-CONDUCTING GLAZE coMPosrTioNs This is a divisional of application Ser. No. 70,404 filed Sept. 8, 1970, now US. Pat. No. 3,658,583.

The present invention relates to an electric insulator having an improved surface electrical stress distribution. More particularly, the present invention relates to a semi-conducting glaze composition capable of firing under a reducing atmosphere, an electric insulator having a semi-conducting glaze coating, and methods for producing the same.

It is necessary for porcelain for electric insulators to have no moisture absorption in order to maintain its dielectric strength at the best condition, and to obtain uniformly reproducible results and high mechanical strength. It has been known that these properties required by an electric insulator are influencedhighly by the firing atmosphere. Generally, in firing under an oxidizing atmosphere, temperature distribution in the firing furnace is non-uniform and it is impossible to fire porcelains having a large wall thickness and further the firing temperature range for producing satisfactory products is much more narrow than the temperature range under a reducing atmosphere, and a bloating phenomenon occurs frequently in the resulting porcelain. On the contrary, the firing under a reducing atmosphere has such merit that the sintering of porcelain is promoted, and electric-insulators having a large wall thickness, such as a heavy duty suspension insulator, and a solid core insulator having a maximum core diameter ofmore than 80 mrn ean be completely sintered to the central portion. Moreover, the temperature distribution in the furnace is uniform, and electric insulators having a stability and a high dielectric strength can be always obtained. Furthermore, it has been known that the mechanical strength of porcelain generally increases in proportion to the amount of crystals formed in the porcelain, and in the production of porcelains having a high mechanical strength, in which crystals are formed during the firing process of porcelain and the amount of the crystals is increased, it is indispensable to effect the firing under a reducing atmosphere. Further, it has been known that when an insulator body using Toseki (Pottery Stones, available in Japan),

1 which is a main raw material for porcelain for electric insulators, is fired, porcelains having no moisture absorption cannot be obtained unless the firing is effected under a reducing atmosphere.

Based on the above described reasons, it is very effective to effect the firing of the porcelain for electric insulators under a reducing atmosphere, and the essential points in the firing under a reducing atmosphere usually effected are as follows. A porcelain is fired under air atmosphere from room temperature to a temperature of 900 l,000C and fired under a reducing atmosphere of l percent CO gas concentration from a temperature of 900 1,000C to a maturing temperature of 1,200 l,3.80C and then cooled under air atmosphere. Of course, the CO gas concentration at the maturing temperature is lowest, and as the temperature is lower, the CO gas concentration is higher.

The surface resistivity of a semi-conducting glaze coating applied on a high voltage insulator should be in the range of 1 100 MQ/sq. For the production of a semi-conducting glaze coating having such a surface resistivity, it has been attempted to add various conducting metal oxides to a conventional ceramic glaze composition. That is,

l. Semi-conducting glaze composition containing ferric oxide as the conducting metal oxide:

This glaze composition can be applied on an electric insulator body having a large wall thickness, which requires the firing under a reducing atmosphere, but it has been known that when the electric insulator applied with this glaze composition is used in a heavily polluted atmosphere, the insulator is liable to be subjected to an electrolytic corrosion. The temperaturecoefficient of resistivity is highly negative, which gives susceptibility to thermal instability, and consequently an electric insulator having the semi-conducting glaze over substantially its entire surface fails due to the thermal instability. The appearance of this glaze is unfavouthis semi-conducting glaze composition is rably black. 2. Semi-conducting glaze composition containing titanium oxide as the conducting metal oxide:

When this glaze composition is applied on a prepared insulator body and fired under a reducing atmosphere, the titanium oxide is reduced and the surface resistivity of the resulting semi-conducting glaze coating is less than 100 MQ/sq. However, titanium oxide glazes are damaged by discharges, resulting in loss of conductivity due to reoxidation of the titania. Furthermore, the preparation condition, particularly the firing condition must be controlled strictly, and the glazing process of complicated, and the commercial production is fairly ,restricted. Consequently, this glaze composition cannot be used commonly.

3. Semi-conducting glaze composition containing stannic oxide (SnO,) and antimony pentoxide (Sb O,,) as the conducting metal oxide:

When this glaze composition is applied on a prepared insulator body, the semi-conducting glaze coating has a high resistance against electrolytic corrosion, a low negative temperature-coefficient of resistivity and an adequate coefficient of thermal expansion, and the coating surface is grey or white, Therefore, this glaze composition is particularly suitable for the production of high voltage insulators, which require a high mechanical strength. Examples of these semi-conducting glaze compositions are disclosed in British Pat. No. 982,600; 1,098,958 (corresponding to US. Pat. No. 3,368,026) and 1,112,765. In all the above patents, a blue tin oxide, which is obtained by calcining a mixture of 99 mol percent of SnO and 1 5 mol percent of Sb O at a temperature of l,000 1,200C or by calcining a mixture of 99.5 95 mol percent of SnO and 0.5 5 mol percent of Sb O at a temperature of 1,200 1,300C, is mixed with a conventional ceramic glaze composition, and the resulting mixture is applied on the surface of a ceramic article and fired to obtain a semiconducting glaze coating on the surface of the ceramic article. However, these methods are applicable to a firing of electric insulators under an oxidizing atmosphere, but are not applicable to electric insulators having a large wall thickness which require firing under a reducing atmosphere, because the surface resistivity of the resulting semi-conducting glaze coating is more than 100 Mil/sq, and the surface is blistered considerably.

According to the present invention, when a calcined material composed of tin oxide and antimony oxide and a conventional ceramic glaze compositon are pulverized and mixed and the mixture is melted to prepare a fritted material, and the fritted material and a conventional ceramic glaze composition are again pulverized and mixed, particles, in which the electrically conductive oxides are coated with frit having conventional ceramic glaze composition, can be obtained. Accordingly, even if a prepared insulator body glazed with the semi-conducting glaze composition is fired under a reducing atmosphere, in each of the particles the firing atmosphere is shut out by the covered frit layer and the electrically conductive oxide in the center portion of each particle is not influenced by the reducing atmosphere, and a semLconducting glaze coating, in which the appearance and the electrical conductivity are not deteriorated, can be formed.

Moreover, this semi-conducting glaze composition is easily applicable to an electric insulator having a large wall thickness, which requires the firing under a reducing atmosphere and has hitherto been impossible by the conventional method, and even in the electic insulators having a large wall thickness, the electric properties can be remarkably improved.

The details and objects as well as the characteristic features of the present invention will become more apparent and more readily understandable by the following description and the appended claims when read in conjunction with he accompanying drawings.

For a better understanding of the invention, reference is taken to the accompanying drawings, wherein:

FIG. 1, A and B are flow sheets showing comparison of a conventional method for producing an electric insulator by using a conventional semi-conducting glaze composition with the method of the present invention by using the semi-conducting glaze composition of the present invention;

FIG. 2 is a graph showing a relation between the ratio of tin component to antimony component in the production of the semi-conducting glaze composition of the present invention and the properties of the coating of the semi-conducting glaze after applied on an electric insulator body and fired under a reducing atmosphere;

FIG. 3 is a graph showing a relation between the firing condition in the fritting process and the properties of the coating of semi-conducting glaze after applied on an electric insulator body and fired under a reducing atmosphere;

FIG. 4 is a graph showing a relation between the mixing ratio of the electrically conductive oxide to the conventional ceramic glaze composition in the fritting process in the production of semi-conducting glaze composition and the properties of the coating of the semiconducting glaze after applied on an electric insulator body and tired under a reducing atmosphere; and

FIG. 5 is a front view, partly in section, of a solid core line post insulator provided with a semi-conducting glaze coating according to the present invention.

The present invention will be explained hereinafter with reference to one preferred embodiment.

. FIG. 1A shows a conventional method and FIG. 1B shows a method of the present invention. Referring to FIG. 1B, at least one tin oxide, such as stannic oxide (SnO and. metastannic acid (H SnO in an amount of 85 94 mo] percent calculated as SnO is mixed with at least one antimony oxide, such as metallic antimony (Sb), antimony trioxide (Sb O and antimony pentoxide (Sb O in an amount of 6 l5 mol percent calculated as Sb O and the resulting mixture is calcined at a temperature of l,O()O 1,300C under air or an oxidizing atmosphere (hereinafter, this process is referred to as a calcination process) and then pulverized. 25 45 precent by weight of the resulting powders is mixed with 55 percent by weight of conventional ceramic glaze composition for insulators consisting of commonly used glaze materials, such as feldspar, dolomite, talc, calcite, kaolin, clay and quartz sand, and the resulting mixture is melted at a temperature of 1,200 1.400%: under air or an oxidizing atmosphere to prepare a fritted material (hereinafter, this process is referred to as a fritting process). Not less than 70 percent by weight of this fritted material and not more than 30 percent by weight of at least one of clay, kaolin, bentonite or conventional glaze compositions are pulverized and mixed by means of a trommel to prepare a semi-conducting glaze slip. This slip is a semiconducting glaze composition suitable for applying on an insulator body having a large wall thickness, which requires the firing under a reducing atmosphere. When this slip is applied on the surface of a prepared insulator body by a conventional process, for example, a spraying process or a clipping process, and the glazed insulator body is subjected to a conventional firing under a reducing atmosphere which is commonly used for firing porcelains for electric insulator, a semi-conducting glaze coating having a surface resistivity of 1 MQ/sq. and a grey orwhite color is formed on the insulator body surface.

According to the calcination process, antimony component is doped into tin component to develop electrical conductivity. When the calcination temperature is lower than l,OOOC under air or an oxidizing atmosphere, the antimony component cannot be completely doped into the tin component, and when the glaze containing these calcined material is applied on a prepared insulator body, the properties of the semi-conducting glaze coating are instable. While, when the calcination temperature is higher than 1,300C, the antimony component volatilizes rapidly, and the desired tin component doped with antimony component cannot be obtained, and consequently when the glaze containing these calcined material is applied on a prepared insulator body, the surface resistivity of the semi'conducting glaze coating is higher than 100 MQ/sq. Therefore, it is necessary that the calcination process should be effected at a temperature of l,000 1,300C under an oxidizing atmosphere. Furthermore, if the calcination process is effected under a reducing atmosphere, for instance, in the presence of gaseous CO or gaseous H the tin component is reduced. at a temperature higher than about 1,000C to form metallic tin, which volatilizes, so that the desired tin component doped with antimony component cannot be obtained. While, if the temperature is less than 1,000C, the antimony component cannot be completely doped into tin component, and the desired semi-conducting glaze composition cannot be obtained. Therefore, the calcination process should be effected under an oxidizing atmosphere.

In the fritting process, the elctrically conductive oxides are coated with frit having conventional ceramic glaze composition, and this fritting process is the essential feature of the present invention. By this fritting process, the electrically conductive oxides are coated with the above described frit to form a fritted material. Ac-

cordingly, even when a semi-conducting glaze composition prepared from this fritted material is fired under a reducing atmosphere which is indispensable for an electric insulator having a large wall thickness, these Therefore, it has been found that it is preferable to i i use a tin component in an amount of 85 94 mol percent calculated as SnO and to use an antimony component in an amount of 6 mol percent calculated as cent by weight, the surface resistivity of the resulting semi-conducting glaze coating increases rapidly and exceeds I00 MQ/sq. While, the semi-conducting glaze composition prepared from the fritted material alone lowers somewhat in workability, but can be used. Accordingly, the amount of at least one of clay, kaolin, bentonite or other conventional ceramic glaze compositions is preferably not more than percent by weight.

The following examples are given in illustration of this invention and are not intended as limitations thereof.

EXAMPLE I SnO and Sb O were mixed in a ratio as shown in the following Table l and the resulting mixture was calcined at l,200C for 2 hours in air in an electric furnace. The calcined material was pulverized to a particle size of 325 meshes under. 30 percent by weightof the resulting powder and 70 percent by weight of a conventional ceramic glaze composition were mixed and melted at l,300C in air to prepare a fritted material. 97 percent by weight of the resulting fritted material and 3 percent by weight of kaolin were mixed and pulverized to a particle size of 325 meshes under by means of a trommel to obtain a semi-conducting glaze slip. The slip was applied on a prepared insulator body, and the glazed insulator body was subjected to a firing under a reducing atmosphere at a maturing temperature of 1,2603C and a maximum CO gas concentration of 6.l percent to obtain a fired insulator body, which had a surface condition and a surface resistivity as shown in FIG. 2.

As seen from FIG. 2, when the ratio of Sb O is less than 6 mol percent, blisters are formed on the surface of the semi-conducting glaze coating, and when the ratio is more than 15 mol percent, the surface resistivity of the semi-conducting glaze coating is higher than I00 Mfl/sq., and consequently the aimed properties cannot be obtained.

oxides are not reduced and the semi-conducting glaze 5 Sb O More particularly, in the case of a tin compocomposition can develop completely its excellent propnent of 88 92 mol percent (as SnO and an antimony erty. component of 8 12 mol percent (as Sb O the sur- When a semi-conducting glaze composition is preface of the semi-conducting glaze coating is good and pared from not less than percent by weight of the the surface resistivity is stable.

, TAlLz Mol% sb o, 1 3 5 7 l0 I2 15 16 I8 20 fritted material and not more than 30 percent by weight EXAMPLE 2 of at least one of clay, kaolin, bentonite or conventional ceramic glaze compositions, the resulting semi- 29 A mixture of 92 mo] percent of snoz and 8 mol pep f l glaze composmm 9 3 of cent of Sb O was calcined at I ,100C for 2 hours in air y an w Improve m wor a an e in an electric furnace, and the calcined material was glaze P prepared m the fmtad mammal pulverized to a particle size of 325 meshes under. 35 alone. Moreover, the semi-conducting glaze coating IS .2 percent by weight of the resulting powder was mixed 5 g gg ggg g gzgf g 5323 123fl gg g gggg with 65 percent by weight of a conventional ceramic as the amount of theabove clay kaolin bentonite of g g fi tgwfrsl'lmng mlxture was Subecte tot e o owlngt e rrttmg processes. 'i g cezfimlcdglaze lm g g In one of the processes, the mixtures were melted at owever, w en e sai amoun is more an per- 30 1,0000, 1,1000 1,2000 1,3000, 1,4000 and 1,500oc for 2 hours in air in an electric furnace, respectively. In another process, the mixtures were melted at l,l00, l,200, l,300 and 1,400C for 2 hours under a reducing atmosphere of 3 percent CO gas concentration, respectively.

percent by weight of each of the resulting fritte materials and 5 percent by weight of a conventional glaze composition were mixed and pulverized by means of a trommel to obtain ten semi-conducting glaze slips. Each of the resulting slips was applied on a prepared insulator body and the glazed insulator body was subjected to a conventional firing under a reducing atmosphere at a maturing temperature of 1,280C and a maximum CO gas concentration of 6.4 percent to obtain a fired insulator body. The surface resistivities of the fired insulator bodies are shown in FIG. 3.

As seen from FIG. 3, when the firing temperature in the fritting process under air atmosphere is lower than l,200C or higher than l,400C, the surface resistivity is higher than MQ/sq., and consequently such firing temperature ranges are not preferable. Further, it has been found that when the firing in the fritting process is effected under a reducing atmosphere, the surface resistivity is always higher than 1,000 MQ/sq., and consequently the firing under a reducing atmosphere is not preferable.

an electric furnace, to obtain fritted materials. Each of the fritted materials was pulverized to a particle size of 325 meshes under by means of a trommel to obtain a semi-conducting glaze slip.

Each of the semi-conducting glaze slips was applied on a prepared insulator body, and the glazed insulator bodies were subjected to a conventional firing under a reducing atmosphere at a maturing temperature of l,280C and a maximum CO gas concentration of 6.4 percent to obtain fired insulator bodies. The surface r'esistivities of the fired insulator bodies are shown in FIG.

As seen from FIG. 4, unless the electrically conductive oxide and the conventional ceramic glaze composition are used in amounts of 25 45 percent by weight and 55 75 percent by weight respectively, the surface resistivity is higher than 100 MQ/sq., and therefore the semi-conducting glaze composition beyond this range is not preferable.

0.33 mm, the other portion 2 of the insulator body was glazed with a conventional light-grey nonconducting glaze, and then the external periphery 3 of the lower end was sanded. The above treated insulator body was fired under a conventional reducing atmosphere at a maturing temperature of 1,260C and a maximum CO gas concentration of 0.6 percent.

The surface resistivity of the semi-conducting glaze coating after the firing was within a range of 1 1.0 14.8 MQ/sq., and the surface had a good condition and a light-grey colour.

Then, a base metal fitting was cemented to the external periphery 3 and the corona starting voltage test was made and the voltage observed was about 45 KV.

EXAMPLE 5 Semi-conducting glaze slips as shown in the following Table 3 were prepared and these slips were applied on the entire surface ofprepared snspensioninsulator bo- A mixture of 90 mol percent of SnO and mol percent of Sb Q was calcined at l,200C for 2 hours in air in. the electric furnace, and the calcined material was pulverized to a particle size of 325 meshes under. 35 percent by weight of the resulting powder was mixed with 65 percent by weight of a conventional ceramic glaze having a molar composition of 0.40 of KNaO,

' 0.30 of CaO, 0.30 of MgO, 0.75 of A1 0 and 6.00 of SiO and the resulting mixture was fired at 1,250C for 2 hours in air in an electric furnace to obtain a fritted material, which was then pulverized to a particle size of 325 meshes under. 90 percent by weight of the pulverized fritted material and 10 percent by weight of the above-mentioned ceramic glaze composition were mixed by means of a trommel by adding water in such an amount that the water content was 42 percent by weight to prepare a semi-conducting glaze slip. The glaze slip was applied on the top portion 1 of insulating part of a 34.5 KV line post insulator body as shown in FIG. 5, the maximum core diameter after the firing being 80 mm, in a thickness of the glaze layer of 0.30

T LE 2 7: by weight Electrically l0 l5 2s conductive oxide Conventional 9O 85 80 75 7O 6O 55 4 50 45 ceramic glaze composition 1 dies for extrahigh voltage overhead transmission lines EXAMPLE 4 having a diameter of 320 mm in a thickness of the glaze layer of 0.27 0.32 mm, and the glazed insulator bodies were fired under a conventional reducing atmosphere at a maturing temperature of 1,290C and a maximum CO gas concentration of 5.8 percent. After completion of the firing, cap and pin were cemented to the fired insulator body, and the surface resistivity, surface condition and withstand voltage test under polluted condition was made. The obtained result is also shown in Table 3. The test is effected in the following way, that is, a constant voltage is applied to the specimen which has been dried after polluted to degree of pollution in terms of salt deposit density of 0.21 mglcm and then artificial fog is produced and the maximum voltage that the specimen will endure without flashover or failure due to thermal instability is examined.

As seen from Table 3, the suspension insulator body TABLE 3 Preparation Condition of Semi-conducting Glaze Present invention Conventional (b) (c) (d) method Combination of electrically conductive oxides SnO ferric oxide H SnO titanium dioxide Sb and chromic Sb.,O oxide 9 TAl B lZ E FJ -Continued Preparation Condition of Semi-conducting Glaze Present invention Conventional (a) b 1) method M iitin g ratio of electrically conductive Hm oxides (mol%. calculated value) SM): 88 88 88 88 Fc,0,: 60

Sb,0, 12 12 12 12 TiO,: 25

Calcination condition (in air, in an electric furnace) temperature(C) l 150 l 150 l 150 l 150 not calcined time (hour) 2 2 2 2 Particle size of pulverized calcined -3 2S 325 325 -325 material (mesh) Ceramic glaze composition for the preparation of fritted material (molar composition) KNaO 0.30 0.30 0.35 0.35 CaO 0.50 0.50 0.45 0.45 MgO 0.20 0.20 0.20 0.20 A1 0, 0.60 0.60 0.65 0.65 SiO, 5.00 5.00 5.00 5.00

Mixing ratio of calcined material to ceramic glaze composition in'the production of fritted material by weight) calcined material .35 35 35 ceramic glaze composition 65 65 65 Firing condition in fritting process (in air, in an electric furnace) I temperat'ure(C) 1350 1350 1350 1350 time (hour) 2 2 2 2 Particle size of pulverized fritting 325 325 325 325 material (mesh) Compounding recipe in the production Electrically conductive of semiconducting glaze slip by oxide: 25%; Ceramic weight) glaze composition,

fritted material 100 80 KNaO: 0.30

kaolin O 0 20 O CaO: 0.20 clay O 20 O 0 MgO: 0.50 bentonite 0 0 0 5 A1 0 0.65

Properties of Suspension Insulator Surface resistivity (Mil /sq.) 5-l2 32-42 28-40 15-26 18-39 Surface condition color grey light grey light grey grey black condition good good good good good 12.0 (failed due to thermal instability at l2.5KV)tZ,l/64

16 .5 (flashed over 16.0 flashed over 16.0 (flashed over 16.5 (flashed o'ver Withstand voltage under polluted at 17.0KV) at 16.5KV) at 16.5KV) at 17.0KV)

condition per unit insulator (KV) l l otei mark indicates the oxide used.

The clay, kaolinite and bentonite used ir fthe 556% TABLE 4 Table 3 had chemical compositions as shown in the following Table 4.

' The ceramic glaze having a molar composition of 0.2

55 Chemical compositions by weight) of clay, kuolinite and henlonite used in Example 5 0.5 of KNaO, 0.2 0.6 of CaO, less than 0.3 of MgO, Clay Kaolinilc Bcnwniw 0.5 0.9 of A1 0 and 4.0 9.0 of SiO is preferably used in the present invention. Ignition LOSS 852 2-22 Although in the abovementh)nedERWeTEe 60 A: 5 33180 12:86. semi-conducting glaze composition according to the 5285 3: 3-32 is: invention is applied on the top portion 1 of prepared 2 linepost insulator bodies and a conventional non- K20 0070 2.70 2.01 conducting glaze composition is applied on the other N310 portion 2 of the insulator vbodies, according to the pres- 65 9997 99' 98,71

ent invention the semi-conducting glaze composition of scale and heavy duty suspension insulators and solid core insulators having an excellent semi-conducting glaze coating can be produced. Moreover, the pollution withstand voltage of the resulting insulators is improved. Consequently, transmission lines to be used under severe pollution condition can be designed. Furthermore,'the resulting insulators have beautiful grey or white appearance.

While, several examples have been herein disclosed, it is obvious that various changes can be made without departing from the spirit and scope of the invention as set forth in the appended claims. Further, it is to be unone antimony oxide selected from the group consisting of Sb, Sb O and Sb O at a temperature of l,O00 l,300C under an oxidizing atmosphere; pulverizing the calcined mixture; mixing 25 45 percent by weight of the pulverized mixture with 55 75 percent by weight of a conventional ceramic glazecomposition; melting the resulting mixture at a temperature of l,200 1,400C under an oxidizing atmosphere; pulverizing the melt to obtain a fritted material; and mixing not less than v percent by weight of the resulting fritted material with not more than 30 percent by weight of at least one member selected from the group consisting of clay, kaolin, bentonite and conventional ceramic glaze compositions.

2. The method as claimed in claim 1, wherein said electrically conductive oxide is composed of 88 92 mol percent, calculated as SnO of at least one tin oxide selected from the group consisting of SnO and H SnO and 8 e 12 mol percent calculated as Sb O of at least one antimony oxide selected from the group consisting of Sb, Sb O and So O 3. The method as claimed in claim 1, wherein said tin oxide is SnO and said antimony oxide is Sb O 4. The method asclaimfed in claim 2, wherein said tin oxide is SnO and said antimony oxide is Sta- 0 

2. The method as claimed in claim 1, wherein said electrically conductive oxide is composed of 88 - 92 mol percent, calculated as SnO2, of at least one tin oxide selected from the group consisting of SnO2 and H2SnO3 and 8 - 12 mol percent calculated as Sb2O5, of at least one antimony oxide selected from the group consisting of Sb, Sb2O3 and Sb2O5.
 3. The method as claimed in claim 1, wherein said tin oxide is SnO2 and said antimony oxide is Sb2O5.
 4. The method as claimed in claim 2, wherein said tin oxide is SnO2 and said antimony oxide is Sb2O5. 